Interconnections was written by Radia
Perlman, a leading networking expert. She wrote this book intending
to help people understand networking problems and provide a
reference to relevant issues. She didn't simply present a
collection of texts condensed from the literature and specs;
instead, by keeping an open mind, she documented many techniques,
both good and bad, in a carefully arranged manner with an expert's
insight. It is one of the best books available on networking
today.

People surf the Internet, from which they find an explosion
of sources of human wisdom and civilization. Behind the scenes of
the Internet are specialized boxes and protocols. These boxes are
called bridges, routers, switches and hubs, and are connected via
various communication channels, through which they talk with each
other in predefined languages known as protocols. Writing a book to
describe this area comprehensively is a daunting task. We haven't
yet seen many good books in the field.
Interconnections is excellent in its coverage
of issues in both layer 2, that is, the data-link layer on which
bridges are operating; and layer 3, the network layer on which
routers are operating. It presents a broad view of the techniques
to connect computers together. The book gives an excellent
introduction to all these issues.

The first edition of this book, published in 1992, has been a
best-seller and acclaimed as a classic on the topic. Since then,
tremendous progress has been made in internetworking technology.
Traditional companies are feeling pressure from new start-ups in
the networking arena. More people are eager to learn about
networking. It is great to see a new edition of
Interconnections published at this time. The
second edition is not simply an update, but a competent rewrite of
most chapters, with a few exceptional chapters remaining largely
unchanged. This new edition places more emphasis on networking
designs and covers issues not found in the first edition, such as
hubs, fast packet forwarding, autoconfiguration, multicasting and
protocol design folklore.

This book is comprehensive for general networking concepts
and techniques, including obsolete techniques in legacy systems
such as X.25 and up-to-date advances such as multicast routing. The
author intentionally puts various networking techniques (both good
and bad) in one place to help people understand not only why we do
something in this way but also why not in that way, in hopes that
future networking designers can learn from these lessons and avoid
the same mistakes. Given a problem, the author doesn't simply list
a well-accepted solution, but often provides several alternatives.
This helps reveal the differences, advantages and weaknesses of
various solutions. For example, in Chapter 9 the author summarizes
almost all well-known network layer address structures, whether
widely or seldom used: IP, IPX, IPv6, CLNP, AppleTalk and
DECnet.

Interconnections are implemented mainly by bridging and
routing. The book discusses issues around these two interconnection
approaches. It also mentions connections by devices like hubs,
which are simple repeaters and widely used in offices. The best
part of this book is the section on bridging. One of the author's
greatest contributions, the spanning tree algorithm, was made in
this area. Routing is complex. The author also did an excellent job
in summarizing this issue. Most chapters are devoted to routing.
Compared with other books on routing techniques, this one covers
more general topics and has more author's comments. Moreover,
Perlman's book is outstanding in that it deals with both bridging
and routing in one text. Bridging techniques rarely found anywhere
else are described in depth.

Confusion in networking terminology often arises in the
networking industry. This is due to the complexity of the problems
and, historically, the lack of consensus. The author makes complex
topics understandable by using simple words and examples. The
author tells us the word “switch” means nothing but “fast”
bridge or router or some hybrid, better used as a generic term for
a box that moves data. She also relates anecdotes, which make the
text more entertaining as well as more informative.

Like many other books on networking, this one starts with an
introduction to the well-known, seven-layered OSI reference model.
The first chapter is characterized by discussions of a few
networking properties which are important in network
designs.

I found chapters 2, 3 and 4 to be the most interesting part
of the book, as with the first edition. They are excellent
descriptions of bridging techniques. The author describes
transparent bridging and her invention of the spanning tree
algorithm in detail. As an alternative (although not popular)
solution, source routing bridges are introduced in Chapter
4.

Chapters 6 to 15 are about network layer (that is, layer 3)
techniques. These chapters cover introductions to almost all major
issues about moving data around internets, or simply routing. In
these chapters, readers can find a wide range of information on
networking techniques such as network service models (like
connectionless and connection-oriented, best-effort and reliable
services), address structures (like IPv4 and IPv6) and routing
protocol data packet formats, and unicast and multicast routing
protocols (like RIP, BGP, OSPF, ISIS, PNNI, DVMPR, MOSPF and
PIM-SM).

Chapter 16 is an abstraction of the author's PhD dissertation
on a technique for a high degree of network robustness.

Chapter 17 and 18 summarize the book. Chapter 17 compares
bridges and routers. It further clears the confusion between terms
of bridges, routers and switches. Chapter 18 is completely new in
the 2nd edition. The author illustrates some good protocol design
philosophy by relating some folklore. I thought it was the most
entertaining chapter of the book.

The Internet infrastructure, consisting of interconnected
devices and information sources, is a must for Linux to flourish.
Linux, in turn, has been making ever-growing contributions to
interconnection technology. Although not a single word about Linux
can be found inside, Interconnections should
benefit anyone inside and outside the Linux community, beginner and
expert, developer and manager, who is curious about what happens
inside networking boxes and wants to obtain a solid knowledge of
networking.

He Zhu
(hezhu@yahoo.com) is interested in system
software and networking. He is currently working for Bell Labs,
NJ.

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